PSI - Issue 48

2

H. Vidinha et al. / Procedia Structural Integrity 48 (2023) 135–141 Vidinha et al / Structural Integrity Procedia 00 (2023) 000 – 000

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Nomenclature DIC

Digital image correlation

FRP Fibre-reinforced polymer GFRP Glass fibre-reinforced polymer N f

Number of cycles to fatigue failure Scanning electronic microscope

SEM

σ max Maximum cyclic stress ε I First principal strain

Moreover, with the demand for reducing greenhouse gas emissions, projects for marine transportation vessels are becoming more weight sensitive, which obligates optimized designs and thinner structures, Davies et al., (2001). Accordingly, environmental ageing is crucial in the design process and cannot be neglected. Although fibre reinforced polymers (FRPs) have respectable corrosion characteristics compared to some metals, severe degradation can still occur, compromising structure’ s stability. Moreover, although FRP materials are not susceptible to marine corrosion due toheir chemical stability, they are affected by other degradation mechanisms induced by the presence and absorption of surrounding moisture, Kafodya et al. (2015). This degradation is commonly caused by multiple factors, including matrix microcracking, debonding between the fibre/matrix interface, and ply delamination, which can lower the strength and ultimately induce the failure of the structure, Schutte (1994). Numerous studies have been carried out to assess how composite materials are impacted by hostile environments, Kootsookos & Mouritz (2004) or Mourad et al. (2010) or Bian et al. (2015) or Branco et al. (2021). Ellyin & Rohrbacher (2000) showed that moisture absorption in glass/epoxy composites causes plasticisation and swelling, resulting in residual stresses that compromise the mechanical performance of exposed composite structures. Kennedy et al. (2016) conducted a study on the effect of prolonged immersion in seawater on the fatigue properties of glass fibre-reiforced polymers (GFRPs). Their findings revealed a considerable reduction in the fatigue strength of the composite after prolonged exposure to water. More specifically, after 1000 cycles, the fatigue strength decreased by 20% compared to dry conditions. Similarly, after 1 million cycles, the reduction in fatigue strength under wet conditions was 8%. In 2019, Gibhardt et al. (2019) conducted a study where they found a significant reduction of fatigue life under cyclic tension – compression. The authors validated the hypothesis that the fibre-matrix interphase is a major factor in terms of durability. Another study conducted by Pacheco et al. (2012) confirmed that the microstructure of the interphase plays an important role in mechanical strength and in promoting damage by facilitating the diffusion process. Considering the existing literature on the mechanical effects of seawater exposure, this study aims to explore how seawater-induced physical changes affect the fatigue strength of GFRP. Consequently, notched specimens were used in fatigue tests, with immersion times ranging from 0 to 230 days. Additionally, digital image correlation (DIC) was utilized to track the damage evolution during cyclic loading, and high-quality scanning electron microscopy (SEM) was employed to assess the fracture surfaces of specimens that failed due to fatigue. 2. Experimental procedure This study used composite laminates prepared by hand lay-up made of glass fibre (1195P) with an epoxy resin Biresin® CR122 and a hardener Biresin® CH122-3, both supplied by Sika. The laminates were composed by 12 plies arranged according to the layout [0º,45º,90º,45º,0º,90º]s and the final geometry of the plates were 330×330×2.3 mm 3 . For the experiments, rectangular specimens were obtained from those plates with dimensions of 165×22.5 mm 2 . A specialized twist drill bit designed for composites was used to drill a central hole measuring 5 mm in diameter. After manufacture, the samples were immersed in seawater for 230 days to evaluate the effect of exposure time on their mechanical properties. The samples were weighed regularly to assess the water absorption rate during the immersed period. The seawater used to age the specimens was obtained directly from the port of Figueira da Foz, Portugal, which is exposed to the waters of the Atlantic Ocean. The fatigue tests were conducted at room temperature using a sinusoidal waveform, a